types of DNA repair

Minor DNA damage

An altered base

DNA cross-linking

May block DNA replication or transcription

Single strand breaks

Base excision repair, nucleotide excision repair, mismatch repair

Double strand breaks

Non-homologous end joining, homologous recombination

Base excision repair (BER)

Used where a single base has either been modified by or excised by hydrolysis leaving an abasic size. (hydrolytic damage / depurination)

Specific DNA glycosylase cleaves the sugar-base bond to delete the base

Residual sugar-phosphate reside is removed by enzymes and the gap is filled by DNA polymerase and DNA ligase

Can only fix small errors

DNA mismatch repair (MMR)

Repaired erroneous insertion, deletion, mis-incorporation of bases that occurs during DNA replication and recombination (G1/S)

Repair of DNA damage on a single DNA strand

Nucleotide excision repair (NER)

Repairs bulky, helix-distorting DNA lesions

(Used for UV induced T-dimers)

DNA is cleaved and about 30 nucleotides containing the damaged site are discarded

Important in G1 but not restricted to it

Homologous recombination (HR)- mediated DNA repair

Requires a homologous intact DNA strand to be available as template

Operates in S and G2 phase

Repairs DNA lesions affecting both DNA strands

Non-homologous end joining (NHEJ)

No template needed because broken ends are fused together

DNA ligase IV rejoins ends

NHEJ is always available but most important in G1 before replication happens

Repair of DNA lesion affecting both DNA strands

Genetic material can be missing but usually wont cause a disease

DNA damage and repair